Drive circuit for brushless DC fan motor

ABSTRACT

A drive circuit for a brushless DC fan motor for rotating a rotor (permanent magnet) by a rotating magnetic field produced by turning on/off the flow of electric current to field coils by a signal to control terminals of switch elements in which switch sections are each connected in series with the field coils. When connecting capacitors for absorbing a current spike in parallel with the switch sections of the switch elements, resistors are each connected between the field coil and switch section, through which the capacitors are connected in parallel with the switch sections. A discharge current of the capacitors which absorbed a current spike when turning on the switch elements is passed via the resistors, thereby eliminating a current spike.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a brushless DC fan motorsuitably radiating heat generated in a housing of electronic equipment,more particularly, to a drive circuit therefor.

[0003] 2. Description of the Related Art

[0004] For example, in electronic equipment having a large number ofelectronic parts housed in a relatively narrow housing, such as OA(office automation) equipment including a personal computer, a copyingmachine and so on, the housing may be filled with heat generated fromthe above electronic parts, thereby breaking the electronic parts byheat.

[0005] Therefore, an air blower is provided in a wall surface or aceiling surface of the housing of the electronic equipment, to which afan motor is attached, thereby externally exhausting the heat in thehousing.

[0006] While a brushless DC fan motor is often used as this type of fanmotor, a conventional drive circuit for driving such a brushless DC fanmotor will be shown in FIG. 4.

[0007] In the drawing, reference numeral 41 denotes a drive circuit fora brushless DC fan motor, here, which indicates a drive circuit in abrushless DC fan motor 42 of a two-phase unipolar drive type. Symbols +and − indicate the anode and the cathode of a DC source, respectively.

[0008] As shown in the drawing, the brushless DC fan motor 42 includesfield coils (motor coils) L1 and L2. The field coils L1 and L2 areprovided at a stator (not shown) and the flow of electric current isswitched alternately by switch elements of the drive circuit 41, whichare NPN transistors T3 and T4 here, to establish a rotating magneticfield. A rotor (not shown) of the fan motor 42 includes a permanentmagnet, the permanent magnet being rotated following the rotatingmagnetic field to rotate the rotor.

[0009] The drive circuit 41 is constituted by a control circuit CC,resistors R1 to R9, PNP transistors T1 and T2, Zener diodes ZD1 to ZD4,diodes D1 and D2, and the above-mentioned transistors T3 and T4.

[0010] The control circuit CC receives a signal from a hall element 43for detecting the location of the rotor (permanent magnet) and outputs asignal for performing an on-off control of the transistors T3 and T4. Inother words, after the output signal from the control circuit CC thepolarity of which has been reversed and the signal has been amplified bythe transistors T1 and T2, it is input to the transistors T3 and T4,thereby performing the on-off control of the transistors T3 and T4.

[0011] Accordingly, the flow of electric current to the magnetic fieldsL1 and L2 is alternately switched in accordance with the location of theaforesaid rotor to produce a rotating magnetic field, and the rotor isrotated as described above, thereby externally exhausting the heat inthe housing.

[0012] In such a fan motor, there is a possibility of generating a highvoltage (generating a high voltage periodically) when the electricity isturned off, thereby destroying elements including the transistors T3, T4and so on.

[0013] Accordingly, in the above-described conventional circuit, theZener diodes ZD1 and ZD2, and ZD3 and ZD4 are connected between acollector and the base of each of the transistors T3 and T4,respectively, thereby preventing the generation of the high voltage.

[0014] However, even if the Zener diodes ZD1 and ZD2, and ZD3 and ZD4are connected, in the conventional circuit shown in FIG. 4, a currentspike flows toward the collector of the transistors T3 and T4 when theelectricity of the field coils L1 and L2 is turned off, thereby causingnoise.

[0015] Also, upon starting the motor, a current spike flows toward thecollector of the transistors T3 and T4, causing noise.

[0016]FIG. 5 is a view of the conventional circuit for reducing acurrent spike to the transistors T3 and T4, as described above.

[0017] Referring to FIG. 5, the same elements as those in FIG. 4 aredenoted by the same reference numerals. Here, capacitors C1 and C2 areconnected in parallel between the collector and an emitter of each ofthe transistors T3 and T4 in FIG. 4, respectively.

[0018] With such a construction, in the conventional circuit shown inFIG. 4, a current spike which tends to flow toward the collector of eachof the transistors T3 and T4 flows toward the capacitors C1 and C2,respectively, and is then absorbed. Accordingly, the noise due to theabove current spike can be reduced.

[0019] However, the effect of preventing noise was not enough even inthe conventional circuit shown in FIG. 5.

[0020] More specifically, the current spike which tends to flow towardthe collector of each of the transistors T3 and T4 can be securelyabsorbed by the capacitors C1 and C2. Consequently, the noise producedby current spike flow toward the collector of each of the transistors T3and T4 can be prevented.

[0021] However, when turning on the transistors T3 and T4 after thecurrent spike has flowed toward the capacitors C1 and C2, a currentdischarge spike of each of the capacitors C1 and C2 flows toward thecollector of each of the transistors T3 and T4, respectively.

[0022] In the conventional circuit shown in FIG. 5, the noise producedby such discharge current flow poses a new problem, which was brought tothe fore when chip capacitors (not shown) were used as the capacitors C1and C2.

[0023] Accordingly, one of the solving means is to not use the chipcapacitors as the capacitors C1 and C2. However, the chip capacitors(not shown) are often used as the capacitors C1 and C2 in order torespond to a requirement for miniaturization of a drive circuit board.At any rate, conventionally, reduction of the noise caused by thecurrent discharge spike from the capacitors C1 and C2 is earnestlyrequired to realize a low noise level.

SUMMARY OF THE INVENTION

[0024] The present invention is made in consideration of the aforesaidrequirement, accordingly, it is an object of the present invention toprovide a drive circuit for a brushless DC fan motor for reducing thevarious types of noise such as noise produced by a current spike causedby turning off the electricity of a field coil or upon startup of amotor, furthermore, the noise caused by a current discharge spikeproduced when a current-spike absorbing capacitor discharges and forrealizing a low noise level.

[0025] In order to achieve the above object, in accordance with thepresent invention, a drive circuit for a brushless DC fan motorcomprises: a field coil provided at a stator; a switch element in whicha switch section formed between a power supply side terminal and aground side terminal is connected in series with the field coil; a rotorhaving a permanent magnet; a capacitor connected in parallel with theswitch section for absorbing a current spike; and a resistor insertedbetween the field coil and the switch section for discharging thecapacitor, wherein the flow of electric current to the field coil isturned on or off by a signal to a control terminal of the switch elementto rotate the rotor by a rotating magnetic field produced by the fieldcoil; and wherein the capacitor is connected in parallel with the switchsection via the resistor.

[0026] The drive circuit for a brushless DC fan motor according to thepresent invention further comprises a Zener diode connected between anode of the field coil and the capacitor and the control terminal of theswitch element or between both ends of the capacitor in a reversedirection of the polarity therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a circuit diagram of an embodiment of a drive circuitaccording to the present invention.

[0028]FIG. 2 is an explanatory view of the operation of the samecircuit.

[0029]FIG. 3 is a circuit diagram of an essential part of anotherembodiment of the same circuit.

[0030]FIG. 4 is a view of a conventional circuit.

[0031]FIG. 5 is a view of another conventional circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Embodiments of the present invention will be describedhereinbelow with reference to the drawings.

[0033]FIG. 1 is a circuit diagram of an embodiment of a drive circuitfor a brushless DC fan motor according to the present invention.

[0034] In the drawing, reference numeral 41 denotes a drive circuit fora brushless DC fan motor, which indicates a drive circuit in a brushlessDC fan motor 42 of a two-phase unipolar drive type. Symbols + and −indicate the anode and the cathode of a DC source, respectively.

[0035] As shown in the drawing, the brushless DC fan motor 42 includesfield coils (motor coils) L1 and L2. The field coils L1 and L2 areprovided at a stator (not shown) and the flow of the electric current isswitched alternately by switch elements of the drive circuit 41, whichare NPN transistors T3 and T4, to establish a rotating magnetic field. Arotor (not shown) of the fan motor 42 includes a permanent magnet, thepermanent magnet being rotated following the rotating magnetic field torotate the rotor.

[0036] The drive circuit 41 is constituted by a control circuit CC,resistors R1 to R11, PNP transistors T1 and T2, Zener diodes ZD1 to ZD4,diodes D1 and D2, and the above-mentioned transistors T3 and T4. The PNPtransistors T1 and T2 are used for inverting the polarity and foramplification, the diodes D1 and D2 are used for preventing backflow ofcurrent, the resistors R10 and R11 are used for discharging thecapacitors, and the capacitors C1 and C2 are used for absorbing acurrent spike.

[0037] Here, the control circuit CC includes an amplifier section AMPfor amplifying a signal from a hall element 43 for detecting thelocation of the rotor (permanent magnet), a controller section CTR towhich a signal from the amplifier section AMP is input for generating asignal for performing on/off control of the transistors T3 and T4, andan output section OC for providing the signal from the controllersection CTR to the transistors T1 and T2.

[0038] The base of the PNP transistor T1 is connected to a first outputterminal OUT1 of the control circuit CC via the resistor R3 andconnected to the anode + of the DC source via the resistors R2 and R1and a backward diode D2 in this order. A collector of the transistor T1is grounded via the resistors R4 and R5 in this order.

[0039] The base of the PNP transistor T2 is connected to a second outputterminal OUT2 of the control circuit CC via the resistor R9 andconnected to a node of the resistors R1 and R2 via the resistor R8. Acollector of the transistor T2 is grounded via the resistors R6 and R7in this order.

[0040] Emitters of the transistors T1 and T2 are connected to a commonnode of the resistors R2 and R8.

[0041] The base (control terminal) of the NPN transistor (switchelement) T3 is connected to a node of the resistors R4 and R5, and theemitter (ground side terminal) is grounded. A collector (source sideterminal) of the transistor T3 is connected to the anode + of the DCsource via the resistor R10, the field coil L1 and the backward diode D1in this order. In other words, according to the present invention, theresistor R10 for discharging the capacitor is connected between thefield coil L1 and a switch section formed between the collector and theemitter of the transistor T3.

[0042] The base (control terminal) of the NPN transistor (switchelement) T4 is connected to a node of the resistors R6 and R7, and theemitter (ground side terminal) is grounded. A collector (source sideterminal) of the transistor T4 is connected to a node of the diode D1and the field coil L1 via the resistor R11 and the field coil L2 in thisorder. In other words, according to the present invention, the resistorR11 for discharging the capacitor is connected between the field coil L2and a switch section formed between the collector and the emitter of thetransistor T4.

[0043] The Zener diodes ZD1 and ZD2 are connected in series in the samedirection and inserted between a node of the field coil L1 and theresistor R10 and the base of the transistor T4 in a reverse orientationof the node of the field coil L1 and the resistor R10.

[0044] Also, the capacitor C1 is connected between the node of the fieldcoil L1 and the resistor R10 and the emitter of the transistor T3. Inother words, according to the present invention, the capacitor C1 isconnected in parallel (with the switch section) between the collectorand the emitter of the transistor T3 via the resistor R10 fordischarging the capacitor.

[0045] The Zener diodes ZD3 and ZD4 are connected in series in the samedirection and inserted between a node of the field coil L2 and theresistor R11 and the base of the transistor T4 in a reverse orientationof the node of the field coil L2 and the resistor R11.

[0046] Also, the capacitor C2 is connected between the node of the fieldcoil L2 and the resistor R11 and the emitter of the transistor T3. Inother words, according to the present invention, the capacitor C2 isconnected in parallel (with a switch section) between the collector andthe emitter of the transistor T4 via the resistor R11 for dischargingthe capacitor.

[0047] Next, the operation of the circuit according to the presentinvention will be described also with reference to FIG. 2.

[0048] The control circuit CC receives the signal from the hall element43 for detecting the location of the rotor (permanent magnet) andoutputs a signal from the output terminals OUT1 and OUT2 for performingthe on/off control of the transistors T3 and T4.

[0049] The output signal from the control circuit CC is input to thetransistors T1 and T2 for inverting polarity and for amplification, thetransistors T1 and T2 performing the on/off control of the transistorsT3 and T4.

[0050] More specifically, when the signal from the output terminal OUT1of the control circuit CC is at a low level, the transistor T1 is turnedon and current flows from the anode + of the DC source to the cathode −of the DC source via the diode D2, the resistor R1, the switch sectionbetween the emitter and the collector of the transistor T1, andresistors R4 and R5.

[0051] Accordingly, the transistor T3 the base potential of which israised is turned on and current flows from the anode + of the DC sourcetoward the cathode − of the DC source via the diode D1, the field coilL1, and the switch section between the collector and the emitter of thetransistor T3, thereby generating a magnetic flux from the field coilL1.

[0052] The above operation will be continued until the signal from theoutput terminal OUT1 of the control circuit CC changes to a high level.

[0053] When the signal from the output terminal OUT2 of the controlcircuit CC is at a low level, the transistor T2 is turned on and currentflows from the anode + of the DC source to the cathode − of the DCsource via the diode D2, the resistor R1, the switch section between theemitter and the collector of the transistor T2, and the resistors R6 andR7.

[0054] Accordingly, the transistor T4 the base potential of which israised is turned on and current flows from the anode + of the DC sourcetoward the cathode − of the DC source via the diode D1, the field coilL2, and the switch section between the collector and the emitter of thetransistor T4, thereby generating a magnetic flux from the field coilL2.

[0055] The above operation will be continued until the signal from theoutput terminal OUT2 of the control circuit CC changes to a high level.

[0056] Here, the low-level signal to the base of each of the transistorsT1 and T2 is alternately output at an appropriate interval and time fromthe output terminals OUT1 and OUT2 of the control circuit CC whichreceived the signal from the hall element 43. Accordingly, electricityis alternately applied to the field coils L1 and L2 similarly and,thereby the field coils L1 and L2 generate a rotating magnetic field.

[0057] Consequently, the rotor of the fan motor 42 rotates as describedabove to externally exhaust heat in the housing.

[0058] The Zener diodes ZD1 and ZD2, and ZD3 and ZD4 reduce thedestruction of elements including the transistors T3 and T4 and so ondue to a high voltage (a high voltage generated periodically) generatedwhen the electricity of the field coils L1 and L2 is turned off).

[0059] More specifically, when the voltage between both ends of theZener diodes ZD1 and ZD2, and ZD3 and ZD4 is raised to a predeterminedvalue or more due to the high voltage upon turning off the electricity,both ends are connected to allow electric current to flow as shown bythe arrows A and A′ in FIG. 2, thereby preventing destruction of theelements including the transistors T3 and T4 and so on in a mannersimilar to the conventional circuits shown in FIGS. 4 and 5.

[0060] The capacitors C1 and C2 pass a current spike toward itself(refer to the arrows C and C′ in FIG. 2), the current tending to flowtoward the collector of each of the transistors T3 and T4 (refer to thearrows B and B′ in FIG. 2) due to a high voltage upon turning off theelectricity for absorbing, thereby reducing the noise, which is similarto the conventional circuit shown in FIG. 5.

[0061] In the circuit of the present invention, as shown in FIG. 1, thecapacitor C1 is connected in parallel with the switch section betweenthe collector and the emitter of the transistor T3 via the resistor R10.The capacitor C2 is also connected in parallel with the switch sectionbetween the collector and the emitter of the transistor T4 via theresistor R11.

[0062] With such a construction, discharge current of the capacitors C1and C2 flowing toward the collector of the transistors T3 and T4 whenthe transistors T3 and T4 are turned on after a current spike has flowedtoward the capacitors C1 and C2 flows via the resistors R10 and R11, asindicated by the arrows D and D′ in FIG. 2. As a result, the currentdischarge spike is reduced, so that noise produced by a steep dischargecurrent is eliminated, thereby realizing a low noise level That issimilar to the case of using chip capacitors (not shown) as thecapacitors C1 and C2.

[0063] Also upon startup of the motor, a current spike flows toward thecollector of the transistors T3 and T4, causing the noise. In this caseas well, a reduced current spike flows by the capacitors C1 and C2 andthe resistors R10 and R11. Consequently, noise caused by a currentdischarge spike can be eliminated, thereby realizing a low noise level.

[0064] In the above embodiment, although the Zener diodes ZD1 and ZD2,and ZD3 and ZD4 are connected between the node of the field coils L1 andL2 and the resistors R10 and R11 and the base of the transistors T3 andT4, respectively, they may be connected between both, ends of thecapacitors C1 and C2, as shown in FIG. 3. Alternatively, the Zenerdiodes ZD1 and ZD2, and ZD3 and ZD4 may be omitted depending on thevalue of the voltage generated when the electricity of the field coilsL1 and L2 is turned off, or the Zener diode may be one provided for eachcapacitor (for example, ZD1 and ZD3).

[0065] Referring to FIG. 3, symbols E and E′ indicate a current spikefrom each of the field coils L1 and L2, respectively. In addition,elements similar or corresponding to those in FIGS. 1 and 2 areindicated by the same reference numerals in FIG. 3.

[0066] In the above embodiments, although an NPN transistor is used as aswitch element, a PNP transistor may be used, alternatively, afield-effect transistor (FET) or the like may be used.

[0067] Furthermore, the brushless DC fan motor is not limited to thetwo-phase unipolar drive type.

[0068] As described above, in accordance with the present invention, thedrive circuit for a brushless DC fan motor comprises: the field coil; aswitch element in which a switch section is connected in series with thefield coil; a rotor having a permanent magnet; capacitor connected inparallel with the switch section for absorbing a current spike; and aresistor inserted between the field coil and the switch section fordischarging the capacitor, wherein the flow of electric current to thefield coil is turned on or off by a signal to a control terminal of theswitch element to rotate the rotor by a rotating magnetic field producedby the field coil; and wherein the capacitor for absorbing the currentspike is connected in parallel with the switch section via the resistor.

[0069] With such a construction, a current spike caused upon turning offthe flow of the electric current to the field coil or upon starting up amotor is absorbed by the capacitor, thereby reducing the noise. Inaddition, a steep discharge current of the capacitor which absorbed thecurrent spike upon turning on the switch element flows via the resistor,producing a flat-peak (not a spike) current, thereby preventing thenoise due to various causes to realize a sufficiently low noise level.

[0070] The drive circuit for a brushless DC fan motor according to thepresent invention further comprises the Zener diode connected betweenthe node of the field coil and the capacitor and the control terminal ofthe switch element or between both ends of the capacitor in a reversedirection of the polarity therebetween.

[0071] With such a construction, the effect of reducing a voltage can beobtained at a wide range (value) for a high voltage generated whenturning off the flow of the electric current to the field coil,decreasing a peak value of electric current flowing to the power supplyside terminal (switch section) of the switch element, thereby enhancingthe effect of reducing the noise.

What is claimed is:
 1. A drive circuit for a brushless DC fan motor,comprising: a field coil provided at a stator; a switch element in whicha switch section formed between a power supply side terminal and aground side terminal is connected in series with the field coil; a rotorhaving a permanent magnet; a capacitor connected in parallel with theswitch section for absorbing a current spike; and a resistor insertedbetween the field coil and the switch section for discharging thecapacitor, wherein the flow of electric current to the field coil isturned on or off by a signal to a control terminal of the switch elementto rotate the rotor by a rotating magnetic field produced by the fieldcoil; and wherein said capacitor is connected in parallel with theswitch section via the resistor.
 2. The drive circuit for a brushless DCfan motor according to claim 1, further comprising: a Zener diodeconnected between a node of the field coil and the capacitor and thecontrol terminal of the switch element or between both ends of thecapacitor in a reverse direction of the polarity therebetween.